.content{
    width: 640px;
    height: 640px;
    margin: 100px auto;
    position: relative;
}
.btn{
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    left: 0;
    background-color: rgba(0,0,0,.3);
    z-index: 2;
}
/* 按钮公共类 */
.btn button{
    background: none;
    border:none;
    background-size: 100% 100%;
    cursor: pointer;
    outline: none;

    position: absolute;
    left: 50%;
    top: 50%;
}
/* 开始按钮 */
.startBtn button{
    width: 200px;
    height: 80px;
    background-image: url(../images/startGame.png);
    transform: translate(-50%,-50%);
}
.pauseBtn{
    /* 暂停按钮刚开始不显示 */
    display: none;
}
/* 暂停按钮 */
.pauseBtn button{
    width: 70px;
    height: 70px;
    background-image: url(../images/pause.png);
    margin-top: -35px;
    margin-left: -35px;
}

/* 遮罩层 */
#snakeWrap{
    width: 600px;
    height: 600px;
    background-color: #225675;
    border: 20px solid #7dd9ff;
    position: relative;
}
/* 蛇头 */
.snakeHead{
    background:url(../images/snake.png) no-repeat;
    background-size:cover;
}
.snakeBody{
    background-color:  #9ddbb1;
    border-radius: 50%;
}
.snakeHead{
    background:url(../images/snake.png) no-repeat;
    background-size:cover;
}
.food{
    background-image: url(../images/food.png);
    background-size: cover;
}